This study is directed towards correlating state transitions in the nicotinic cholinergic receptor with receptor desensitization. The state transitions are reflected in change in affinity for agonists, but not antagonists, and can be examined in the intact cell or in isolated membrane fragments. Functional parameters can only be measured in the intact cell and involve monitoring of agonist induced changes in sodium permeability in cultured cells through the estimation of unidirectional 22 Na ion fluxes. The studies with isolated membrane fragments employ purified membranes from the Torpedo electric organ with its abundance of receptor and the cell culture studies employ the non-fusing muscle cell line BC3H-1.